The long term objective of this proposal is to gain new insights into the treatment or prevention of breast cancer by understanding hormonal control of normal mammary growth and development. GH, acting through GH receptors in the mammary gland, is the pivotal pituitary hormone required for mammary development. Experimental evidence strongly suggests that IGF-I mediates this activity. 1) GH induces IGF-I mRNA within the mammary gland, 2) IGF-I can substitute for the pituitary in mammary development in rats and mice, and 3) mammary development in IGF-I deficient knockout mice is significantly retarded. GH also activates mammary estrogen receptors (ER), offering a rationale for E2 having no independent effect on the immature gland, yet synergizing with GH for full development. To determine the role of GH-induced IGF-I in mammary development and the physiological mechanisms involved, we plan to study effects of GH, PRL, and IGF-I, without or with E2, on biochemical and anatomical parameters of normal mammary development in 2 animal models. One is the hypophysectomized, oophorectomized, sexually immature female rat which permits studies at puberty, and the other is the IGF-I deficient knockout mouse allowing examination of mammary development in the total absence of IGF-I. We hypothesize that GH acts on stromal tissue in the mammary gland, starting a cascade of events including paracrine action of IGF-I on mammary glandular development or induction of cancer when some other factor such as a carcinogen is present. We plan to study effects of IGF-I in development and as a mediator of GH action, determine how E2 synergizes with GH and IGF-I, and localize sites of action of these hormones to specific cells in the mammary gland by in situ hybridization, and immunohistochemistry including immuno-electron microscopy. We suggest that understanding control of normal mammary development will eventually result in novel insights into mammary carcinogenesis.